1. Field of the Invention
The present invention relates to an HST (hydrostatic transmission) including a hydraulic pump unit and a hydraulic motor unit, which are arranged spaced apart from each other.
2. Background Art
The HST includes a hydraulic pump unit operatively driven by a driving power source, a hydraulic motor unit for operatively driving a driving-wheel, operation fluid lines for fluidly connecting the hydraulic pump unit and the hydraulic motor unit so as to form a closed circuit, and a charge line for replenishing the operation fluid to the closed circuit.
An HST of a type in which the hydraulic pump unit and the hydraulic motor unit are arranged spaced apart from each other has been proposed (see e.g., Japanese Laid-Open Patent Publication No. 2003-291674).
This type of HST is effective in that the degree of freedom in designing the working vehicle to which the HST is applied can be enhanced since the hydraulic pump unit and the hydraulic motor unit are respectively arranged at the desired positions.
However, this type of conventional HST has drawbacks in that a free-wheel phenomenon is likely to occur at the driving-wheels, and in that it is difficult for the engine brake operation to effectively apply to the driving-wheels.
The drawbacks of the conventional HST will be described taking a case in which a pair of conduits for fluidly connecting the hydraulic pump unit and the hydraulic motor unit is used as part of the operation fluid lines by way of example.
The flow path resistance of the pair of conduits increases in the HST described above since the hydraulic pump unit and the hydraulic motor unit are arranged spaced apart from each other.
Therefore, when a motor main body of the hydraulic motor unit unintentionally performs a pump action in a time when the working vehicle travels downhill, the suction amount of the operation fluid by the motor main body tends to become shorter than the discharge amount of the operation fluid by a pump main body of the hydraulic pump unit due to the flow path resistance, whereby a negative pressure is created in one of the pair of conduits.
In this case, the fluid is replenished through the charge line to one of the conduits where negative pressure occurs, but the replenishing fluid supplied through the charge line is also subjected to flow path resistance since the charge line is arranged in the hydraulic pump unit in the conventional HST. Accordingly, the negative pressure that occurs in one of the conduits cannot be effectively resolved by replenishing the operation fluid through the charge line.
When negative pressure occurs in one of the conduits, a phenomenon in which the motor main body sucks the air occurs and the amount of air present in the pair of conduits increases, which leads to the free-wheel phenomenon of the driving-wheels and inhibits the engine brake operation.